1. Field of the Invention
The present invention relates to a radiation curable resin composition, which is used for the purpose of obtaining desired diffusion angle, luminance in emission direction, and luminance in peak direction of output light using light from a light source, and which is suitable for production of optical components such as a Fresnel lens sheet having a structure in which a molded layer having a lens function made of a cured resin article, for example, a Fresnel lens layer is formed on a plate- or sheet-shaped plastic substrate, and relates to a Fresnel lens sheet obtained by curing the resin composition coated on the surface of the substrate. Also the present invention relates to a radiation curable resin composition for a Fresnel lens, which exhibits a high elastic modulus and a high refractive index and is superior in adhesion to the plastic substrate and in transparency, wherein a lens layer exhibits excellent shape retention over a wide temperature range and is less likely to chip and crack due to external force, and to a Fresnel lens sheet provided with a Fresnel lens layer made of the radiation curable resin composition.
2. Description of Related Art
As a transmission type screen used in projection TV sets, for example, there has hitherto been known a screen with a configuration in which a Fresnel lens sheet, which converts diffused light from a light source such as a CRT into parallel light beams and transfers the parallel light beams to the user, is used in combination with a lenticular lens sheet which has an effect of distributing light towards only the viewing angle of the user so as to effectively utilize a fixed quantity of light.
As the method of molding a Fresnel lens sheet, there has recently been proposed a method of forming a Fresnel lens layer on a plastic substrate using a radiation curable resin composition and efficiently forming a Fresnel lens sheet only by irradiation with radiation such as ultraviolet light within a short time.
There has been reported an example wherein an epoxy (meth)acrylate resin composition, which exhibits a high elastic modulus and a high refractive index, is used as a radiation curable resin composition to be used in the Fresnel lens layer. However, since the Fresnel lens sheet is sometimes locally deformed by an external force during the manufacturing process or when assembled in an apparatus, the brittleness of the Fresnel lens layer is not sufficiently improved when using an epoxy (meth)acrylate resin composition having a high elastic modulus. For example, there were problems in that chipping, cracking and breakage occurred when a shock is given to a fine shape formed on the Fresnel lens sheet or when the sheet is cut into a desired size.
Furthermore, it has recently been required that the Fresnel lens sheet be superior in adhesion to the transparent plastic substrate to be used as a substrate and also have a good shape retention over a wide temperature range including a high temperature range because the Fresnel lens sheet is used in various environments.
To cope with these required characteristics, Japanese Unexamined Patent Application, First Publication No. Hei 5-287040, Japanese Unexamined Patent Application, First Publication No. Hei 11-60656, Japanese Unexamined Patent Application, First Publication No. Hei 11-171941, and Japanese Unexamined Patent Application, First Publication No. Hei 11-240926 disclose that the hardness, heat resistance and durability can be improved by using an epoxy (meth)acrylate resin composition which exhibits characteristics of a cured article having a high elastic modulus.
Also Japanese Unexamined Patent Application, First Publication No. Hei 11-152317 discloses a composition for a plastic lens having a high elastic modulus, wherein the heat resistance, impact resistance, and dyeability have been improved by using an epoxy (meth)acrylate resin composition containing polybutylene glycol di(meth)acrylate as an essential component.
Also Japanese Unexamined Patent Application, First Publication No. Hei 3-157412 reports an ultraviolet light curable resin composition for a transmission type screen, comprising, as an essential component, a (meth)acrylate having a polyethylene glycol or polypropylene glycol segment containing dissolved alkali earth metal salt or protonic acid therein, and also discloses, as an example, a resin composition having an antistatic function, which is less likely to cause adhesion of dust, prepared by using a resin composition containing epoxy (meth)acrylate.
However, when the Fresnel lens sheet is actually produced in accordance with the method described in Japanese Unexamined Patent Application, First Publication No. Hei 5-287040, Japanese Unexamined Patent Application, First Publication No. Hei 11-60656, Japanese Unexamined Patent Application, First Publication No. Hei 11-171941 or Japanese Unexamined Patent Application, First Publication No. Hei 11-240926, these methods of the prior art hardly reconcile the prevention of chipping and cracks caused by an external force and shape retention over a wide temperature range, although the resulting cured article is superior in hardness and refractive index.
Also the composition for plastic lens described in Japanese Unexamined Patent Application, First Publication No. Hei 11-152317 is a resin composition cured by irradiation with radiation after pouring into a mirror-polished plastic or glass mold, and the cured article must be removed from the mold after curing. In this case, there is no required adhesion to the transparent plastic substrate, which is required when the Fresnel lens sheet is actually produced, and therefore no study of an improvement in adhesion was made.
Also Japanese Unexamined Patent Application, First Publication No. Hei 3-157412 broadly describes compositions of resin composition for a Fresnel lens and concretely discloses an example of a cured article layer having relatively low elastic modulus, but does not disclose a composition which reconciles the prevention of chipping and cracks caused by an external force and shape retention over a wide temperature range in the case in which a high elastic modulus is imparted to the cured article.
An object to be achieved by the present invention is to provide a radiation curable resin composition for a Fresnel lens, which exhibits a high elastic modulus and a high refractive index and is superior in adhesion to the plastic substrate and superior in transparency, wherein a lens layer exhibits excellent shape retention over a wide temperature range and is less likely to cause chipping and cracks due to an outer force, and to a Fresnel lens sheet using the radiation curable resin composition.
The present inventors have intensively researched in order to meet the required characteristics described above and discovered the following. That is, it is made possible to obtain a radiation curable resin composition for a Fresnel lens, which has excellent adhesion to the plastic substrate and is less likely to cause chipping and cracks due to an outer force while maintaining a high elastic modulus, and also exhibits excellent shape retention over a wide temperature range, and a Fresnel lens sheet using the radiation curable resin composition, by using, in an epoxy (meth)acrylate resin composition, which exhibits a high elastic modulus and a high refractive index, an epoxy (meth)acrylate (a), a compound (b) of the general formula (1) described hereinafter, being selected as a specific trifunctional (meth)acrylate capable of imparting good shape retention and processability, and a (meth)acrylate (c) having a molecular weight of 700 or less, at which a high elastic modulus imparted by epoxy (meth)acrylate is not impaired, and an oxypropylene structure, and a monofunctional (meth)acrylate (d) having a specific structure in combination. Thus, the present invention has been completed.
The present invention provides a radiation curable resin composition for a Fresnel lens comprising, as an essential component, an epoxy (meth)acrylate (a) having two or more (meth)acryloyl groups obtained by reacting an epoxy resin having a cyclic structure and an epoxy equivalent per weight of 450 g/eq or more with (meth)acrylic acid; a trifunctional (meth)acrylate (b) represented by the following general formula (1); a (meth)acrylate (c) which is (meth)acrylate of an aliphatic polyhydric alcohol having a molecular weight of 700 or less and having an oxypropylene structure and two or more hydroxyl groups; and a monofunctional (meth)acrylate (d) having a cyclic structure: 
wherein R1 represents a hydrocarbon group having 1 to 5 carbon atoms and R2 represents hydrogen or a methyl group.
Also the present invention provides a Fresnel lens sheet comprising a substrate made of a (meth)acrylic resin containing methyl methacrylate as a main component, a polystyrene resin or a polycarbonate resin, and a resin layer made of a cured article of the above radiation curable resin composition for a Fresnel lens of the present invention, which is formed on the surface of the substrate in the form of a Fresnel lens.
The cured article obtained from the radiation curable resin composition for a Fresnel lens of the present invention exhibits a high elastic modulus and a high refractive index and is superior in adhesion to the plastic substrate and transparency, wherein a lens layer exhibits excellent shape retention over a wide temperature range and is less likely to cause chipping and cracks due to an outer force.
Therefore, when a screen is made using the cured article, the resistance to the contact pressure with a lenticular lens and the resistance to the pressure due to assembly after TV unit setting are improved, thus simplifying the design with resect to handling during screen setting, for example, operability and assembly. Also it is possible to cope with the design of a lens having a shorter focus because of a high refractive index.